Abstract
Development of underground transportation systems consists of tunnels, basement construction excavations and cut and cover tunnels which may encounter existing pile groups during their construction. Since many previous studies mainly focus on the effects of excavations on single piles, settlement and load transfer mechanism of a pile group subjected to excavation-induced stress release are not well investigated and understood. To address these two issues, three-dimensional coupled-consolidation numerical analysis is conducted by using a hypoplastic model which takes small-strain stiffness into account. A non-linear pile group settlement was induced. This may be attributed to reduction of shaft resistance due to excavation induced stress release, the pile had to settle substantially to further mobilise end-bearing. Compared to the Sp of the pile group, induced settlement of the single pile is larger with similar settlement characteristics. Due to the additional settlement of the pile group, factor of safety for the pile group can be regarded as decreasing from 3.0 to 1.4, based on a displacement-based failure load criterion. Owing to non-uniform stress release, pile group tilted towards the excavation with value of 0.14%. Due to excavation-induced stress release and dragload, head load of rear piles was reduced and transferred to rear piles. This load transfer can increase the axial force in front piles by 94%.
Highlights
In dense urban environments where buildings are closely spaced, deep excavation for basement construction and other underground facilities such as mass rapid transit station sand cut-and-cover tunnels is unavoidable
SUMMARY AND CONCLUSIONS This study reports a 3D coupled consolidation numerical analysis investigating pile group responses to an adjacent excavation in saturated silty clay
Based on the ground conditions and geometries, the following conclusions can be drawn: Owing to the degradation of the stiffness of the clay surrounding the piles in the group and the reduction of shaft resistance due to excavation induced stress release, a nonlinear pile group settlement was induced
Summary
In dense urban environments where buildings are closely spaced, deep excavation for basement construction and other underground facilities such as mass rapid transit station sand cut-and-cover tunnels is unavoidable. Apart from field monitoring, a number of centrifuge tests were conducted to investigate the response of single pile [5] and pile group in soft kaolin clay [6] They concluded that the induced bending moment and lateral deflection of piles were highly influenced by distance from wall and pile head condition. Authors in [10] developed design charts to compute the lateral behavior of a single pile adjacent to deep excavation in soft ground They performed two staged analysis considering plane strain conditions and linear elastic soil model. A monitoring section was selected at the transverse centerline of the excavation In addition to this simulation, a pile load test (L) was conducted numerically in “greenfield” conditions (i.e., without excavation) to obtain the ultimate capacity of the pile in silty clay. The obtained working load was applied to the pile in the analysis simulating excavation
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